The tendency in liquid analysis techniques, inter alia as regards biological liquids, is towards ever smaller specimen volumes which are now a few microliters or even a few nanoliters. Various technical and medical factors in the case of biological liquids are helping to boost this trend. Medicine is deriving increasing amounts of information from analysis results, and so it is common for a number of different analyses to be made from a single sample, hence the need to reduce the volumes of liquid needed to make each analysis so that as small as sample as possible may be taken from the patient. The foregoing is applicable more particularly in the case of analyses of biological liquids taken from the newly born or when repeated samples must be taken from a single patient at frequent intervals. In practical terms, increasing the number of analyses causes space problems for laboratories, the only possible solution being to reduce the size of instruments and apparatus.
If such analyses are to continue to give results at the very strict levels of accuracy required for this purpose, the relationship of the liquid volumes mixed in the test tube must be absolutely correct. This is why the pipette is one of the elements governing analysis quality. Pipetts are used to introduce into test tubes the very small amount of liquid to be analyzed and the comparatively much larger amount of dilution liquid and of reagents in the case of liquid reagents.
It is difficult to devise a pipette which can operate very accurately in a ratio of volumes in a range of from 1 to 10 up to 1 to 100 or even 200 or more. One solution proposed for the problem is to use a pipette pumping constant-volume increments, such volume being the unit pumping volume of the pipette (see U.S. Pat. No. 3,679,331). Of course, the repetition rate of the increments must be fairly high if the rate of flow of diluents and reagents, which may often be more than tens of times greater than the rate of flow of the increment, is to permit sufficiently rapid pipetting, for if it is required to dilute a specimen in a volume of liquid of the order of from 100 to 200 times the pipette increment volume, the increments must be provided in a very rapid sequence if the operation is not to last more than a few seconds. Another important consideration is that the inertia of the pipette drive mechanism must be very low to ensure instant starting and stopping of the pipette.
The volume of the increment forming the working unit of the pipette must be reproducible accurately irrespective of the viscosity of the pumped liquid, the ambient termperature and the aging or wear of the pumping elements. These requirements occur frequently with pipettes; for instance, a pipette may be required to intake a few increments of a specimen for analysis and then to discharge a large number of increments for dilution.
The known pumping device, more particularly those driven by rotating motors (see U.S. Pat. No. 3,679,331), fail to meet all the foregoing requirements since systems driven by such motors have too great an inertia to be able to start and stop instantaneously between two increments. They take some time to run up to their normal operating speed and further time to stop. It is virtually impossible for such a device to start, then stop at the end of a single pump increment and change over consecutively and without transition from intake to discharge
An electromagnetically operated pumping device has been suggested (see U.S. Pat. No. 3,819,305). Electromagnetic operation is satisfactory for on/off control such as the opening and closing of valves but cannot provide accurate control of the alternate variation of the volume of a pumping enclosure or chamber because of the amplitude fluctuations inherent in the movement of the moving element of an electromagnet or solenoid, and movement amplitude, which is directly linked with the size of the increments, must remain accurate to .+-.1%, corresponding to an accuracy of something like .+-.0.1 mm.